1. Field of the Invention
The present invention relates to a liquid ink and an ink jet recording apparatus.
2. Description of the Related Art
A printing machine utilizing a printing plate has been used for the manufacture of a print requiring a reasonably large number of prints such as a local advertisement, materials for distribution within an enterprise, or a large poster. In recent years, an on-demand printing machine, which is capable of dealing with diversified needs and compressing the stock, has come to be used in place of such a conventional printing machine. An electrophotographic printing machine using a toner or a liquid toner or an ink jet printer capable of achieving a high quality printing with a high speed is expected to provide a satisfactory on-demand printing machine.
It is known to the art that a solvent system ink containing a pigment and an organic solvent or a solvent system liquid toner is used in the on-demand printing machine, as in the conventional printing machine using a printing plate. In this technique, however, an organic solvent is evaporated in an amount that cannot be neglected in the case of printing images on a reasonably large amount of paper sheets. As a result, the surrounding atmosphere is contaminated with the evaporated organic solvent, which makes it necessary to arrange an exhaust facility or a solvent recovery mechanism.
Incidentally, in the ink jet printer, the solvent system ink can be handled within a closed system until the ink is spurted onto the surface of the printing paper sheet. As a result, it is possible to suppress the contamination problem of the surrounding atmosphere to some extent by taking suitable exhaust measures. However, the ink used in the ink jet printer differs from that used in the printing machine utilizing a printing plate. To be more specific, it is necessary for the ink used in the ink jet printer to have a fluidity required for the spurting of the ink. Therefore, in the technique described above, it is necessary for the solvent concentration in the ink to be set at a sufficiently high level, with the result that it is essentially difficult for this technique to solve the contamination problem of the surrounding atmosphere caused by from the evaporated organic solvent.
It should also be noted that, in the case of using a solvent system ink, the printing surface of the printing paper sheet, etc., seriously effects the quality of the printed images. For example, the printing surface having a permeability tends to cause the printed images to be blurred, and it is difficult to fix the printed images to the printing surface that is not permeable. Further, a reasonably long time is required for the ink layer formed on the printing surface to be dried. Therefore, in the case of forming a thick image on a large printing surface, the image tends to be collapsed because of the fluidity of the ink. In addition, in the technique using a solvent described above, the printing surface tends to be deteriorated by the drying of the ink layer. It follows that it is not necessarily easy to obtain a high quality print by this technique.
A photosensitive ink and a printer system using the same have come to attract attention as a technique effective for overcoming the problems described above. In this technique, a photosensitive ink spurted onto the printing surface is irradiated with light for achieving a rapid photo-curing (=drying without emissions) of the ink. The photosensitive ink used in this technique typically contains a radical polymerizable monomer, a photopolymerization initiating agent, and a pigment.
According to the technique referred to above, it is possible to permit the ink layer to be rendered incapable of being fluidized by the irradiation with light so as to make it possible to obtain a print having a relatively high quality. However, the ink used in this technique contains a large amount of cancer-causing components such as a radical generating agent. In addition, the volatile acrylic acid derivative used as a radical polymerizable monomer is highly irritating to the skin and is highly odorous. Naturally, it is necessary to handle this ink carefully. Further, the radical polymerization is markedly inhibited by the presence of oxygen in the air. In addition, the pigment contained in the ink absorbs the light irradiating the ink layer, with the result that the dose of the light tends to be rendered insufficient in the deep portion of the ink layer. Under the circumstances, the conventional radical polymerizable ink is low in its sensitivity to light and, thus, a very large light exposure system is required for obtaining a high quality print by this technique.
A photo-cationic curable ink is being proposed as an ink that is less affected by the oxygen in the air. However, the conventional photosensitive ink of this type contains a solvent, which gives rise to the problem in terms of the release of the solvent into the environment. In addition, the ink tends to be easily cured spontaneously and change to be insoluble, giving rise to an additional problem that tends to plug ink-jet nozzle. Also another cationic polymerization type photosensitive composition for the coating of a CD-ROM has been reported, which is capable of being spurted in an ink jet system. The photosensitive composition that was actually found to be capable of being spurted by an ink jet system contained as main components vinyl ether and bisphenol A type epoxy resin that is considered to pose a problem in terms of, for example, a cancer generation. It follows that a serious problem remains unsolved in terms of the release of the photosensitive composition into the environment. Further proposed is an ink for ink jet consisting of cationic polymerization monomers having a specified component monomer ratio. The ink for ink jet also contains as an indispensable component a specified vinyl ether compound having a very high volatility, giving rise to a problem similar to that described above. Also, the ordinary vinyl ether compound, when used in combination with a pigment, gives rise to a problem that the compound is poor in polymerizability.
It is also known to the art to use a solid ink for ink jet in place of the liquid ink referred to above. In this technique, an ink that is a solid at room temperature is fluidized by heating, and the fluidized ink is spurted onto the printing surface of a printing paper sheet. In other words, this technique basically forms a non-solvent system and, thus, the environmental problem is suppressed. Also disclosed is an example in which a photosensitivity is imparted to the solid ink jet ink for fixing the ink layer formed after the spurting of the ink. However, a liquid plugging tends to take place easily in the case of using a pigment that is ordinarily dispersed as a coloring component. Also, it is difficult to uniformly disperse the pigment in the ink. Therefore, in this technique, there are the large problems in dispersing and in stabilizing pigments. Therefore, it is necessary to use a special high-cost micro capsule type pigment, or in usual case, coloring component is limited to a dye which tends to be de-colored with time.
On the other hand, in the ink jet system that is originally constructed such that ink droplets are spurted directly onto a paper sheet forming a printing medium so as to form an ink layer on the paper sheet, the printed image formed by the ink layer is generally blurred or the chromatic sensitivity of the printed image is changed by the printing medium. For overcoming these difficulties, proposed is an ink jet recording apparatus in which a photosensitive ink is once spurted onto a primary printing medium, followed by slightly curing the surface of the ink layer with light so as to cause the ink layer to lose fluidity and subsequently transferring the slightly cured ink layer onto a paper sheet or the like forming a secondary printing medium. In the printer and the ink used for this system, however, a subtle control of the light irradiation amount and the irradiating environment is required for obtaining a semi-set ink layer adapted for the transfer onto the secondary printing medium. In addition, the optimum light exposure amount differs depending on the color. It follows that the ink jet recording apparatus referred to above is not adapted essentially for the printing of a color image.
What should also be noted is that, particularly in the case where the printing surface is formed of an absorbent medium, a serious problem is generated that the conventional acrylic photo-curable ink jet ink is unlikely to be set inside the absorbing paper sheet.
Incidentally, the conventional cationic photo-curable ink jet ink gives rise to the problem that the viscosity of the ink is vigorously changed spontaneously. The problem is derived from the situation that, if an acid is once generated in the ink by the deterioration of the ink with time, the ink is unlikely to be deactivated so as to bring about much dark reactions in the ink. The problem is very serious because, if the viscosity is changed in the ink jet ink, a fatal situation such as a disturbance in the flying shape of the ink, a poor printing reproducibility and, in the worst case, a poor spurting of the ink and the ink plugging in nozzle tends to take place.